Hydrogen for fuel cells can be produced by means of fuel processing. In a fuel processor, a hydrocarbonaceous fuel is converted in a hydrogen-rich gas stream that can be used in a fuel cell for the generation of electricity. Typically in a fuel processor, the hydrocarbonaceous fuel is first reacted with oxygen and/or steam by means of catalytic partial oxidation, autothermal reforming, steam reforming, or a combination of one or more thereof to obtain a gas mixture comprising carbon oxides and hydrogen. The thus-obtained gas mixture is subsequently reacted with steam at a water-gas shift conversion catalyst to convert carbon monoxide into carbon dioxide with concurrent production of hydrogen. A gaseous stream comprising hydrogen and carbon dioxide is thus obtained. This stream may be fed to a fuel cell.
Fuel processors that integrate steam reforming of hydrocarbonaceous streams with selective hydrogen removal are also described in the art, for example in WO 02/070402, U.S. Pat. No. 5,938,800, U.S. Pat. No. 6,348,278, U.S. Pat. No. 5,861,137, and U.S. Pat. No. 5,741,474. Such integrated steam reforming/hydrogen separation devices operate at lower temperatures than conventional steam reforming reactors and are not limited by normal equilibrium limitations. In such integrated devices, hydrocarbons are reformed to carbon dioxide and hydrogen according to (in the case of methane):CH4+2H2O→CO2+4H2Advantages of such integrated steam reformers/hydrogen removal devices as compared to fuel processors without integrated hydrogen removal are that a substantially pure stream of hydrogen is obtained and that no separate reaction zone for the water-gas shift conversion is needed.
In WO 02/070402, a fuel processor comprising a membrane steam reforming reactor is disclosed. The membrane steam reforming reactor comprises a fixed bed of steam reforming catalyst and a hydrogen-selective, hydrogen-permeable membrane for removal of hydrogen from the fixed bed. The membrane steam reforming reactor is heated by a flameless distributed combustor.
In U.S. Pat. No. 5,741,474, a system for producing high-purity hydrogen is disclosed. The system comprises a reforming chamber provided with a fixed bed of catalyst for steam reforming and partial oxidation and a hydrogen-separating membrane. The heat possessed by the portion of reaction gas not permeable into the hydrogen-separating membrane and the heat generated by the exothermic partial oxidation are utilised for heating and reforming.
In U.S. Pat. No. 5,861,137 is described a steam reformer comprising a fixed bed of steam reforming catalyst surrounding at least part of a hydrogen-permeable, hydrogen-selective membrane and a fixed bed of catalytic combustion catalyst arranged around at least part of the fixed bed of steam reforming catalyst. The steam reforming bed is heated by the heat generated by the catalytic combustion of reforming byproduct gases.
In the integrated steam reforming/hydrogen removal processes of the prior art, a fixed bed of steam reforming catalyst is used. In a fixed bed of steam reforming catalyst, it is difficult to achieve an even distribution of the heat needed for the endothermic steam reforming reaction. Even distribution of a fixed bed requires special burners for distributed combustion and/or carefully designed steam reforming chambers, e.g. small annular chambers.
It would be desirable to find a process and reactor for the production of substantially pure hydrogen, wherein heat for the endothermic steam reforming reaction is efficiently provided to that reaction, and the carbon dioxide produced by the process is in a concentrated form.